MONTREAL  Power plants in the U.S. midwest and southeast spew a disproportionately large amount of continental air pollution, according to an environmental commission's study released this week.

The Ohio River valley, parts of Indiana, West Virginia and Illinois in the midwest and Tennessee, northern Georgia and Alabama in the southeast are pollution hot spots caused by coal-fired electricity power plants, the study by the Montreal-based Commission for Environmental Cooperation said.

It compared emissions from 1,000 fossil-fuel plants in Canada, Mexico and the United States using 2002 data and found a small percentage of facilities are responsible for most of the pollution.

While the United States is easily the largest continental polluter because of the size of its economy, its share of electricity-producing emissions is disproportionately large owing to its reliance on coal to produce half its power, the report said.

"This report shows that, site by site, coal-fired power plants are the dominant source of harmful air emissions from the electricity sector in North America," says William Kennedy, executive director of the commission.

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The commission was created by the United States, Canada and Mexico under the North American Free Trade Agreement to ensure that environmental laws are observed under the deal.

The commission's 93-page report measured pollutants such as sulfur dioxide, nitrogen oxides, mercury and carbon dioxide, which can cause acid rain, haze, smog, and climate change, as well as toxic mercury found in fish that people eat.

A plant in Monticello, Texas emits the most mercury of the plants listed but the top individual polluters are not all in the United States. A plant in Veracruz, Mexico leads the continental sulfur dioxide list while another in Coahuila, Mexico produces the most nitrogen oxides.

The report dispels notions that Mexico is a large polluter because it is less developed than its continental partners and that Canada is an environmental haven, co-author Paul J. Miller said.

Mexico uses coal for just 8 percent of its electricity and its three coal-fueled plants produce many times less emission per capita than the United States.

Canada gets 60 percent of its electricity from hydropower. However, the country is also home to the largest carbon-dioxide generating plant on the continent, in Nanticoke, Ontario. It spews 23 million tons of carbon dioxide, a greenhouse gas, every year.

Only a few large plants use modern pollution-fighting technology to reduce emissions but that is slowly changing, Kennedy said.

"A number of power plants are currently installing new technologies to reduce pollution, and this report helps set a North American benchmark with which we can show their environmental achievements over time," he said.